Abstract
Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a, total protein and total glycerolipid also decreased under the N-starved condition based on OD730 reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a, whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration.
Highlights
A typical cyanobacterial cell contains the thylakoid membrane in addition to the plasma membrane and the outer membrane
The rapid decrease in phycocyanin content per culture during N starvation (Supplementary Figure S3) suggests active degradation of PBSs, which would contribute to N recycling from the phycobiliproteins and reducing the antenna size of photosystem II (PSII)
Total protein content per culture was unchanged during N starvation (Figure 6B)
Summary
A typical cyanobacterial cell contains the thylakoid membrane in addition to the plasma membrane and the outer membrane. Proteins in PBSs constitute up to 50% of total protein in the cyanobacterial cell under optimal growth conditions and can provide massive nutrients with degradation in response to N starvation (Collier and Grossman, 1992; Görl et al, 1998). Another objective of the active degradation of PBSs may be avoidance of photodamage caused by over reduction of photosynthetic electron carriers due to the low metabolic activity during N starvation (Forchhammer and Schwarz, 2019). Chlorotic Synechocystis cells re-green and almost completely re-establish thylakoid membranes after 48 h of recovery from N starvation
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